[CF-metadata] CMIP6 Sea Ice MIP: Integrated quantities

From: alison.pamment at stfc.ac.uk <alison.pamment>
Date: Fri, 5 Aug 2016 11:10:29 +0000

Dear Dirk,

Thank you for getting back to me so quickly and for clarifying how the terms "area" and "extent" are used in the sea ice community. Clearly I had misunderstood the meaning of "extent" which just demonstrates the need to add a definition!

If indeed it is always the case that extent describes the area of the grid cells containing ice, then I completely agree that is how we should define it in CF. I also agree that using the terms area_extent and volume_extent would be confusing in those circumstances so I am happy to drop the idea.

Usually in CF we do include the word 'threshold' in any name where a threshold has been applied which is why several of us have suggested that approach during the discussion. However, I take your point that having a threshold is fundamental to the concept of sea_ice_extent, if indeed 'extent' means the area of the grid cells, so in this case I'd be OK with not including it in the name. I agree with Martin's comment that, for clarity, a threshold should always be supplied (even if it is zero) so I think we definitely need to write that into the definition of sea_ice_extent.

Please see below for my summary of where I think we are now on the individual names.

Alison wrote:
> > 1. Current CF proposal: sea_ice_extent_in_region
> > CMIP6 short name: siextentn
> > Long name: Sea ice extent North
> > Units: 10^6 km?
> > Description: Total area of all Northern-Hemisphere grid cells that are
> covered by at least 15 % areal fraction of sea ice
> > My suggestion:
> > Standard name:
> area_extent_of_grid_cells_with_sea_ice_above_threshold (canonical units:
> m2)
> > + new standardized region name 'northern hemisphere' which could be
> supplied in a scalar coordinate variable.
> > The standard name definition can be written so as to allow a threshold of
> sea_ice coverage to be specified, in this case 15%, using a scalar coordinate
> variable. If no threshold is specified it is assumed to be zero. This would
> follow the practice adopted for existing names such as
> time_when_flood_water_falls_below_threshold.
> >
> > I appreciate that the existing name sea_ice_extent doesn't have a
> definition, and maybe we could write something that would allow us to use
> it for the aforementioned variable, but I don't like that solution for the
> following reason. The existing name sea_ice_area has the definition '
> "X_area" means the horizontal area occupied by X within the grid cell', i.e. it
> is the area of the sea_ice itself. By analogy, I would expect sea_ice_extent
> to be defined as something like ' "X_extent" means the horizontal area
> occupied by X summed across the horizontal domain of the data variable as
> described by the associated coordinate variables and coordinate bounds or
> a scalar coordinate variable with a standard name of "region" '. However, if I
> have understood Dirk's variable correctly, it is the sum of the area of the
> grid cells that contain at least 15 per cent sea ice, not the area of the sea ice
> itself, and I would favour introducing a new standard name for that concept.
> Plus it gives us the opportunity to mention the threshold.

Dirk wrote:
> We find the definition suggested by Jonathan very helpful and good:
> "sea-ice_extent: Total area of all grid cells in which the sea ice area
> fraction equals or exceeds a threshold. By default the threshold is 15%.
> The threshold can be specified by supplying a coordinate variable or
> scalar coordinate variable with standard_name of sea_ice_area_fraction."

So we will use the existing name
sea_ice_extent (m2).
The definition should be amended to make it a requirement to supply the threshold and to say something about the geographical area over which extent is calculated:
'The term sea_ice_extent means the total area of all grid cells in which the sea ice area fraction equals or exceeds a threshold, often chosen to be 15 per cent. The threshold must be specified by supplying a coordinate variable or scalar coordinate variable with the standard name of sea_ice_area_fraction. The horizontal domain over which sea ice extent is calculated is described by the associated coordinate variables and coordinate bounds or by a coordinate variable or scalar coordinate variable with the standard name of "region" supplied according to section 6.1.1 of the CF conventions.'

OK?

Dirk wrote:
> Regarding the decision on whether the hemispheres should be defined by
> lat/lon, or simply by a new region definition, I don't have a strong
> opinion, and would be happy to follow either common practice or the
> majority.

I have checked the CF conventions document again regarding this point. Section 6.1.1, "Geographic regions", states: "When data is representative of geographic regions which can be identified by names but which have complex boundaries that cannot practically be specified using longitude and latitude boundary coordinates, a labeled axis should be used to identify the regions. We recommend that the names be chosen from the list of standardized region names whenever possible. To indicate that the label values are standardized the variable that contains the labels must be given the standard_name attribute with the value region." In the case of the northern and southern hemispheres, the boundaries are not "complex" and can conveniently be described using the usual coordinate variables, so I think in fact we should stick to doing that for CMIP6. In the definition of the name I have allowed for the use of a 'region' coordinate or scalar coordinate variable because it is conceivable that someone may at some stage wis
h to calculate sea_ice_extent over an irregularly shaped area. However, I now think that we don't need to add northern_hemisphere and southern_hemisphere to the standardized region list. OK?

Alison wrote:
> > 2. Current CF proposal: sea_ice_area_in_region
> > CMIP6 short name: siarean
> > Long name: Sea ice area North
> > Units: 10^6 km?
> > Description: Total area of sea ice in the Northern hemisphere
> > My suggestion:
> > Standard name: sea_ice_area_extent (canonical units: m2)
> > + new standardized region name 'northern hemisphere' or specify
> horizontal domain via coordinate variables.
> > I think the existing name of sea_ice_extent (with added definition as
> above: "X_extent" means the horizontal area occupied by X summed across
> the horizontal domain of the data variable as described by the associated
> coordinate variables and coordinate bounds or a scalar coordinate variable
> with a standard name of "region") can definitely be used for this variable,
> and for added clarity I'm suggesting we also modify the name itself. Thus
> sea_ice_extent would become an alias of sea_ice_area_extent. This name
> would also work for the analogous southern hemisphere quantity in Dirk's
> original list.

Dirk wrote:
> If we were to use the existing term "sea-ice extent" to describe actual
> "sea-ice area", the CF convention would get in conflict with the entire
> scientific literature, as described above. We don't think this would be
> a good idea, and rather suggest to simply follow Jonathan's suggestion
> to use the existing sea_ice_area, which then should be summed over a
> given region. This would also underpin how closely related sea-ice area
> in individual grid cells is to the integrated sea-ice area that we ask
> for here.

Agreed. We should stick with using sea_ice_area, as currently defined, for this quantity.

Alison wrote:
> > 3. Current CF proposal: sea_ice_volume_in_region
> > CMIP6 short name: sivoln
> > Long name: Sea ice volume North
> > Units: 10^3 km?
> > Description: Total volume of sea ice in the Northern hemisphere
> > My suggestion:
> > Standard name: sea_ice_volume_extent (canonical units: m3)
> > + new standardized region name 'northern hemisphere' or specify
> horizontal domain via coordinate variables.
> > We have an existing name sea_ice_volume which does not have a
> definition. I think we should add a definition and by analogy with
> sea_ice_area I suggest it should be ' "X_volume" means the volume
> occupied by X within the grid cell'. To indicate the sum of the volumes
> across a horizontal domain we would then need a new standard name. For
> consistency with sea_ice_area_extent I suggest 'sea_ice_volume_extent'
> for this name. This would be defined as ' "X_volume_extent" means the
> volume occupied by X summed across the horizontal domain of the data
> variable as described by the associated coordinate variables and coordinate
> bounds or a scalar coordinate variable with a standard name of "region" '.
> Again, this name would work for both northern and southern hemispheres.

Dirk wrote:
> We don't think it's helpful to use extent (which has units m?) to
> describe a quantity that has units m?. Hence, as for sea-ice area, we
> feel it's best to use the existing standard name sea_ice_volume, and to
> ask that this should be summed up over a certain region. We also here
> find Jonathan's respective remark very intuitive.

Agreed. So we will use the existing name
sea_ice_volume (m3)
and add the definition:
' "X_volume" means the volume occupied by X within the grid cell.'
OK?

Dirk wrote:
>
> I hope this makes sense - and sorry for the lengthy reply. It's length
> was also born out of the fact that I'll be offline for the coming three
> weeks, and tried to clarify our view in one go.
>
> Alexandra Jahn, who is co-chair of our CMIP6 sea-ice MIP, might have
> additional comments while I'm gone.
>

I appreciate that we may not be able to finalise all the sea ice names during August when many people are on leave. The next update of the standard name table won't take place until September 13th (again due to the holiday season) so we might be able to agree some, if not all, the names by then. We need to expand on the definition text for all the proposed names to bring them into line with existing names so I will prepare a list summarizing the current state of play, with full definitions, ready for you to review once you are back in the office.

Best wishes,
Alison

------
Alison Pamment Tel: +44 1235 778065
Centre for Environmental Data Analysis Email: alison.pamment at stfc.ac.uk
STFC Rutherford Appleton Laboratory
R25, 2.22
Harwell Campus, Didcot, OX11 0QX, U.K.
Received on Fri Aug 05 2016 - 05:10:29 BST